linux-kernel - Re: [PATCH 5/6] sched/fair: Get rid of scaling utilization by capacity

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Date:	Fri, 11 Sep 2015 08:50:01 +0800
From:	Yuyang Du <yuyang.du@...el.com>
To:	Morten Rasmussen <morten.rasmussen@....com>
Cc:	Vincent Guittot <vincent.guittot@...aro.org>,
	Benjamin Segall <bsegall@...gle.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Steve Muckle <steve.muckle@...aro.org>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"daniel.lezcano@...aro.org" <daniel.lezcano@...aro.org>,
	"mturquette@...libre.com" <mturquette@...libre.com>,
	"rjw@...ysocki.net" <rjw@...ysocki.net>,
	Juri Lelli <Juri.Lelli@....com>,
	"sgurrappadi@...dia.com" <sgurrappadi@...dia.com>,
	"pang.xunlei@....com.cn" <pang.xunlei@....com.cn>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 5/6] sched/fair: Get rid of scaling utilization by
 capacity_orig

On Thu, Sep 10, 2015 at 01:10:19PM +0100, Morten Rasmussen wrote:
> > > so it appear to be intended to be using low resolution like load_avg
> > > (weight is scaled down before it is passed into __update_load_avg()),
> > > but util_avg is shifted up to high resolution. It should be:
> > >
> > >         sa->util_avg = (sa->util_sum << (SCHED_LOAD_SHIFT -
> > >                                         SCHED_LOAD_SHIFT)) / LOAD_AVG_MAX;
> > 
> > you probably mean (SCHED_LOAD_SHIFT -  SCHED_LOAD_RESOLUTION)
> 
> Yes. Thanks for providing the right expression. There seems to be enough
> confusion in this thread already :)
 
And yes, it is my bad in the first place, sorry, I did not think it though :)

> > The goal of this patchset is to be able to scale util_avg in the range
> > of cpu capacity so why don't we directly initialize it with
> > sa->util_avg = SCHED_CAPACITY_SCALE;

Yes, we should, and specifically, it is bacause we can combine the
resolution thing for util% * capacity%, so we only need to use the
resolution once.

> > and then use
> > 
> >  sa->util_avg = (sa->util_sum << SCHED_CAPACITY_SHIFT) / LOAD_AVG_MAX;
> > 
> > so we don't have to take care of high and low load resolution
> 
> That works for me, except that the left-shift has gone be PeterZ's
> optimization patch posted earlier in this thread. It is changing
> util_sum to scaled by capacity instead of being the pure geometric
> series which requires the left shift at the end when we divide by
> LOAD_AVG_MAX. So it should be equivalent to what you are proposing if we
> change the initialization to your proposal too.

I previously initialized the util_sum as:

sa->util_sum = LOAD_AVG_MAX;

it is because wihout capacity adjustment, this can save some multiplications
in __update_load_avg(), but actually if we do capacity adjustment, we must
multiply anyway, so it is better we initialize it as:

sa->util_sum = sa->util_avg * LOAD_AVG_MAX;

Anyway, with the patch I posted in the other email in this thread, we
can fix all this very clearly, I hope so. I did not post a fix patch,
it is because the solutions are already there, it is just how we make it 
look better, and you can provide it in your new version.

Thanks,
Yuyang
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